In an integrated circuit, the transistors are produced in and on a semiconductive active region, for example of silicon, surrounded by an electrically insulating region, for example a trench filled for example with silicon dioxide.
The fact of producing an MOS transistor in an insulating region intrinsically causes a compression-stressed active region to be obtained through the presence at its periphery of the insulating region. Also, while a compression-stressed active region favors the efficiency of a PMOS transistor, it by contrast causes the efficiency of an NMOS transistor to be degraded, notably in terms of carrier mobility.
Moreover, the production of fast transistors demands small channel lengths and widths and the structures generally produced exhibit a significant density, which leads to active region dimensions that are very small, even minimal for the technology concerned.
It is therefore extremely difficult, even impossible, to increase the dimensions of the active regions of the NMOS transistors in order to relax their compression stresses, given the density sought for the structures produced.